1. Field of the Invention
This invention relates to a gas separator and in particular to a gas separator for use as an inline, downhole tool for oil and gas well drilling and servicing.
2. Brief Description of the Prior Art
As described in the Latos et al U.S. Pat. No. 6,138,757, there are occasions in the oil and gas industry when a gas is pumped down a well with a liquid. Coiled tubing deployed jetting services are commonly performed in depleted wells using energized fluids—typically nitrogen and water. Underbalanced operation with energized fluids reduces the potential for well damage and helps to transport fluids and cuttings to surface. When nitrogen and water are jetted as a two-phase fluid, the jet expands as it leaves the nozzle, reducing the jet impact pressure. Two-phase flow in the jet nozzle may also be sonically choked—limiting the jet discharge velocity and effectiveness. Moreover, fluid jets dissipate rapidly in the surrounding wellbore fluid. All these factors combine to reduce the effectiveness of a two-phase jet.
Removal of the gas from the fluid stream would enhance the performance of jetting for well servicing. A single phase water jet has higher density and stagnation pressure than a mixed-phase jet and would be more effective than a two-phase jet. Under conditions found in oil and gas well service operations, the gas cut in the fluid discharge from the separator should be less than 1 vol % to ensure effective jetting.
Shrouding the jets with the separated gas would reduce jet dissipation and increase the effective range of the jet. Many well service operations required that the jetting tools pass through small diameter tubing and obstructions before cleaning larger diameter tubing, downhole equipment in side-pocket mandrels or openhole wellbores; increased jetting range will increase the effectiveness of jetting tools compared to single-phase fluid jetting for these applications.
The use of energized fluid with a gas separator will also boost the differential pressure and hydraulic power of the jet by reducing bottomhole circulating pressure. Increased pressure and power will allow erosion of harder material such as mineral scale, cement and rock, while increased power will improve erosion rates.
An effective gas separator would maintain high efficiency over a relatively high range of inlet gas fractions. In a common application, sufficient nitrogen is added to reduce the bottomhole pressure to 50% of hydrostatic. Under these conditions compressed gas makes up 20 to 60% of the volume fraction of the flow inside the coil. The volume fraction of gas entering the separator may vary substantially during a single run due to changes in pressure and temperature as the operating depth of the tool increases.
The Latos et al patent (supra) describes a downhole phase separator for coiled tubing using a cyclonic separator design. This tool provides less than 5% gas cut for a supply fluid with 30% to 40% gas content. Cyclonic separators are used to swirl fluid flow through a set of vanes. This approach generates very high radial accelerations, which provide the separation forces. In small diameter tools, the high flow rate generates high turbulent mixing forces that overcome the separation forces and limit separation performance.
Rotary gas separators are commonly used in two-phase production to prevent gas from entering electric submersible pumps. The rotary gas separator is powered by the pump shaft and spins at 3500 or 1750 rpm depending on the electric motor and power supply. The system includes an inducer to pressurize the two-phase flow entering the separator. The flow enters a shrouded vane section where the flow spins and the water or oil moves to the outside due to centrifugal forces. The shroud rotates with the vanes reducing turbulence in the separator. A crossover manifold at the top directs the fluid flow to the pump and the gas flow back into the well annulus. The claimed gas cut is less than 10% for a wide range of flow rates and gas/liquid flow ratios,
Inline rotary gas separators are also used in pipelines to remove small volumes of condensate from the gas flow. This style of separator uses a stator to induce swirling flow inside of a drum which includes rotor vanes in the gas flow. The rotor provides power to spin the drum. This type of separator is designed to remove all fluid from the gas stream as opposed to providing a low gas cut in the fluid.
Yahiro et al in U.S. Pat. No. 4,047,580 disclose a method for shrouding a submerged jet by introducing compressed air through the outer annular ring of a concentric jet nozzle. The air shroud increased the range of the jet by a factor of four. The construction of annular gas nozzles is complex, particularly for high-pressure fluid jetting.